Think above is correct on adding time to the map times you estimate. 7 hrs if you have never 'traveled more than an hour' previously is pushing it by a lot. At 10mpg, how far can you go before refueling?
After towing an RV of some kind over 35 years, I think you will be getting tired way before 7 hours, and you won't average more than 55 over the day.
And it will be a lot longer going home....
My wife and family used to make the trips like you are planning--when we were a lot younger, and needed to pile a lot into 2 weeks. Hope you have a great trip.